The present invention generally relates to the field of thermoelectric heat exchangers, which may function either as a heater or a cooler. More particularly, the present invention relates to an improved design of a thermoelectric cooler (TEC), including power system design and packaging design, which result in compactness, increased efficiency, and increased reliability.
TEC""s perform the same cooling function as freon-based vapor compression or absorption refrigerators and air conditioners. In all such units, thermal energy is extracted from a region, thereby reducing its temperature, then rejected to a xe2x80x9cheat sinkxe2x80x9d region of higher temperature. While freon based systems utilize the gas vaporization and compression cycle to perform cooling, thermoelectric coolers utilize the temperature difference that is created across a semiconductor thermocouple when voltage is applied.
A conventional cooling system contains three fundamental partsxe2x80x94the evaporator, compressor and condenser. The evaporator or cold section is the part where the pressurized refrigerant is allowed to expand, boil and evaporate. During this change of state from liquid to gas, energy (heat) is absorbed. The compressor acts as the refrigerant pump and recompresses the gas to a liquid. The condenser expels the heat absorbed at the evaporator plus the heat produced during compression, into the environment or ambient. Vapor-cycle devices have moving mechanical parts and require a working fluid, while thermoelectric elements are totally solid state.
Solid state heat pumps have been known since the discovery of the Peltier effect in 1834. In the Peltier effect, a voltage applied to the junction between two dissimilar metals creates a temperature difference between the two metals. This temperature differential can be used for cooling or for heating.
The devices became practical only recently, however, with the development of semiconductor thermocouple materials. TEC thermocouples are made from two elements of semiconductor, primarily Bismuth Telluride. The semiconductor is heavily doped to create an excess (n-type) and a deficiency (p-type) of electrons. The junction between the n-type and the p-type is a semiconductor thermocouple. At the cold side, energy (heat) is absorbed by electrons as they pass from a low energy level in the p-type semiconductor element, to a higher energy level in the n-type semiconductor element. The power supply provides the energy to move the electrons through the system. At the hot side, energy is expelled to a heat sink as electrons move from a high energy level element (n-type) to a lower energy level element (p-type). Heat absorbed at the cold side is pumped to the hot side at a rate proportional to current passing through the circuit and the number of couples.
These thermocouples, connected in series electrically and in parallel thermally, are integrated into thermoelectric modules. The thermoelectric modules are packaged between metallized ceramic plates to afford optimum electrical insulation and thermal conduction with high mechanical strength in compression. Thermoelectric modules can be mounted in parallel to increase the heat transfer effect or can be stacked in multistage cascades to achieve high differential temperatures. Solid state cooling is relatively simple compared to some of the classical technique using a compressor because there are no moving parts. These devices have the capability to be either heating systems or cooling systems depending on the direction of the current. Thermoelectric modules are divided into a hot side and a cold side, and are typically attached to heat sinks, creating a heat exchanger for use in a TEC.
Development of TECs has enabled the production of commercial miniature solid state air conditioners for cooling enclosures for devices such as electronics lasers, computers, scientific and medical equipment, as well as other similar equipment. Conventional cooling systems for enclosures remove the heat from one place (usually termed a hot spot) and blow the heat somewhere else in the enclosure until it is eventually vented or otherwise conducted/radiated outside. A common technique for cooling is through the use of an exhaust fan that draws outside air (often through filters) through the enclosure. However, certain electronics applications are sealed in an enclosure from the outside environment. This typically dictates the use of a heat exchanger for cooling because a heat exchanger can control the internal temperature of the enclosure without exchanging air between the enclosure and the outside environment. A TEC works well in many of these cooling applications.
However, these TEC""s have some disadvantages. Moisture reaching the thermoelectric modules or the electrical components can reduce reliability. The cooling surface of the TEC often condenses out moisture from the air. The presence of even small droplets of water can cause damage to the thermoelectric modules and this may reduce the operational life of the device and the efficiency of the system. Also, in commercial applications of TECs, the units may be exposed to dust, dirt and water (rain or deliberate wash-down water from cleaning purposes). This exposure to dust, dirt, and water may decrease the reliability and efficiency of the system. In some cases, the units are exposed to acid or chemical attack. Other units require protection from explosive chemicals. Therefore, a TEC, which seals (so as to be highly moisture resistant) the thermoelectric modules would be very desirable. Additionally, a TEC which seals electrical components would be very desirable.
Also, moisture travelling between the hot side and cold side of the TEC may reduce system efficiency by allowing heat to transfer between the hot side and the cold side of the TEC. Also, any moisture placed on the hot side of the TEC (for example by wash-downs, etc.) may penetrate into the cold side of the TEC. This may lead to damage of the devices contained in the enclosure or potentially damage the TEC itself. Therefore, a TEC, which seals (so as to be highly moisture resistant) between the hot side and cold side of the TEC would be very desirable.
Another disadvantage of conventional TEC""s is that they are typically designed with a relatively small cooling capacity. Because of this relatively small cooling capacity, it is important to maximize the thermal isolation between the hot side and cold side of the TEC.
Any transfer of heat from the hot side to the cold side will reduce system performance and efficiency. Any thermal load on the TEC may affect its efficiency. There are generally two, but not limited to two, broad classifications of heat that must be removed from the enclosure. The first is the real, sensible, or active heat load. This is the load that is intended to be cooled. This load could be the I2R load of an electrical component, the load of dehumidifying air, or the load of cooling objects.
The other kind of load is often referred to as the parasitic load. This is the load due to the fact that the object is cooler than the surrounding environment. This load can be comprised of conduction and convection of the surrounding gas, thermal leak through insulation, conduction through wires, waste heat generated from the TEC""s own internal electrical components, condensation of water, and in some cases formation of ice. Regardless of the source of these parasitic loads, they all have potential to affect TEC efficiency.
Thermal loads from the energy dissipation of the TEC""s electrical components may become important and effect operational efficiency if not properly designed. Any airflow or moisture flow between the hot side and the cold side of the cooling system may also reduce overall performance. Therefore, a TEC with improved thermal isolation, improved sealing between the hot and cold side, and/or improved design regarding parasitic loads would be desirable.
Another disadvantage of conventional TEC""s is the size. TEC""s may utilize numerous thermoelectric modules and consume relatively high power, which in certain applications may exceed 800 watts. Most potential industrial/commercial users want standard 120VAC/230VAC power operable equipment. However, thermoelectric modules typically require low voltage, high current DC power. This requires a converter to change 120VAC or 230VAC to low voltage DC. Power conversion using a transformer, diode bridge and smoothing capacitor is a possible choice. However, these conventional devices are large, heavy and not portable in power levels of 300-1000 watts. Use of a transformer/bridge-capacitor power converter adds too much weight and bulk to be commercially acceptable for a compact unit. Standard switching supplies provide better power-to-weight ratios, but they present packaging and sealing problems. Switching supplies offer reduced size and bulk, but are not offered in packages suitable for integration into an air conditioner package. The power supply should also give a DC power with minimal AC ripple. Any AC component on the DC may be detrimental. Additionally, the power system should be lightweight, small, with a flat format and still deliver 600 or more watts. Therefore, a thermoelectric power supply with low AC ripple, low weight, compact in size, and with a flat format would be desirable.
Some conventional TEC""s have a remotely mounted power supply, with the associated electrical components located outside of the housing of the TEC. However, the disadvantage with these TEC""s is that require separate mounting of the power supply and the user must electrically connect the remotely mounted power supply to the TEC. Therefore, a self-contained TEC would be desirable. Self-contained means that the power supply is mounted within the housing of the TEC.
Another disadvantage of conventional TEC""s is that they may operate inefficiently with conventional control systems. Because the performance of a thermoelectric module varies with temperature, conventional control systems may cause the TEC to operate at an inefficient level. In order to maintain a high level of performance efficiency and to avoid the cost of a larger power supply, it is valuable to adjust the power supply using a power control circuit to maximize the cooling that the TEC supplies for a given design and a given set of operating conditions. It is also important to limit the power to the safe operational limit of the thermoelectric module.
It is well known that thermoelectric modules characteristically have an impedance that varies with both the temperature of the hot side of the thermoelectric module and with the temperature difference between the hot side and the cold side of the thermoelectric module. Conventional control systems for TEC""s vary greatly but can be generally considered in two groups: Open Loop and Closed Loop, or manual and automatic respectively. Regardless of the method of control, the easiest device parameter to detect and measure is temperature. Therefore, the cold side (or hot side in heating mode) is usually used as a basis of control. The controlled temperature is compared to some reference temperature, typically the ambient or opposite face of the TEC. In the Open Loop method, an operator adjusts the power supply to reduce the error to zero. The Closed Loop method accomplishes this task electronically. However, because both of these methods typically output a constant voltage, thermoelectric module may operate at an inefficient voltage level. Therefore, it is desirable for a compact thermoelectric cooler to control the output power level for the maximum level of cooling.
Another disadvantage with TEC""s is reliability. TEC""s are sometimes used in harsh environments which may decrease their reliability. Because there is only one power supply, if the one power supply fails, the entire TEC fails. This may cause overheating in the enclosure, potentially damaging equipment. Therefore, it is desirable to design a more fault tolerant TEC.
The present invention is directed to a TEC with moisture resistant barriers around the thermoelectric modules, around the electrical components of the power system, and between the hot side and the cold side of the TEC. This is provided by completely sealing the electrical components and the thermoelectric modules. The seals are achieved with at least one of a sealant, a gasket, and blind fastener holes. To seal the thermoelectric modules, a sealing frame is also used.
Additionally, the present invention is directed to providing a sealing system that inhibits the penetration of moisture between the hot and cold sides of the TEC. This moisture resistance is provided by completely sealing the hot side of the heat exchanger from the cold side.
The present invention is also directed to providing a TEC with increased thermal isolation between the hot side and cold side of the TEC. Increased thermal isolation is provided by completely sealing the hot side of the heat exchanger from the cold side. The sealing design also minimizes any airflow between the hot and cold side of the cooling system, increasing thermal isolation and efficiency. Thermal isolation is also provided by including insulation between the hot side and cold side of the TEC and sufficient spacing between the hot side heat sinks and the cold side heat sinks. It is additionally provided by the design of the power system and packaging techniques in order to minimize the thermal losses and heat contribution from the electrical components. This is provided by designing the system to draw the heat from the heat generating components to the hot side of the TEC, rather than the cold side.
In addition, the present invention is directed to a compact design. A compact design is provided by the use of a DC to DC active power supply and packaging techniques. A DC to DC active power supply avoids the need for a large, heat producing transformer. A DC to DC active power supply therefore, reduces the size of a TEC, and also increases the efficiency. The use of a DC to DC active power supply with a flat format allows packaging techniques that exhaust the heat generated from electrical components to the hot side of the TEC, minimizing the amount of parasitic load. The use of a flat DC to DC active power supply also minimizes the amount of space required to for the electrical components.
The present invention is also directed to maximize the cooling for a given TEC design. The design of a power control circuit provides for maximum cooling of the TEC. This is accomplished by a control circuit that varies the power input to the thermoelectric modules based on the temperature of the hot side of the thermoelectric module and the hot side of the TEC (ambient temperature).
The present invention is also directed to providing increased TEC reliability by supplying two or more DC to DC active power supplies. Each DC to DC active power supply is connected to one or more thermoelectric modules. In this method, even if one DC to DC active power supply fails, several of the thermoelectric modules will still receive power and continue to operate.
The present invention is directed to a compact, self-contained thermoelectric cooler including, a housing having a hot side and a cold side, at least one thermoelectric module disposed between the hot side and the cold side, and a power supply assembly within the housing. The power supply assembly includes a DC to DC active power supply. A mounting frame is disposed between the hot side and the cold side. The mounting frame may also have a mounting flange formed over the outer periphery of at least two sides of a planar body of the mounting frame, and that extend outside of the housing.
The present invention is directed to a compact, self-contained thermoelectric cooler further including a power pack cutout in the mounting frame. A power pack heat sink having a base portion and a plurality of fins, is mounted on the hot side of the mounting frame with the base portion proximate to the power pack cutout. A gasket is attached to the cold side of the mounting frame proximate to the power pack cutout. A power pack cover comprising a base, having four sides extending from a peripheral edge of the base to an outer edge, the base and the four sides defining a cavity, is secured to the gasket, with the outer edge contacting the gasket. The mounting frame, the gasket, and the power pack cover form a barrier between the hot side and the cold side. A plurality of electrical components mounted on the base portion of the power pack heat sink and extending through the power pack cutout are located on the hot side of the barrier. A cover seal may be disposed over the outer edge of the power pack cover.
The present invention is directed to a compact, self-contained thermoelectric cooler further includes a hot side cover attached to the hot side of the mounting frame. The hot side fan has at least one fan opening in the hot side cover. The at least one hot side fan is mounted to the hot side cover, proximate to the fan opening. The compact, self-contained thermoelectric cooler also includes a cold side cover attached to the cold side of the mounting frame. The cold side cover has at least one fan opening in the cold side cover. The at least one cold side fan is mounted to the cold side cover, proximate to the fan opening.
The present invention is directed to a compact, self-contained moisture resistant thermally isolated thermoelectric cooler including at least one moisture resistant barrier, around either the at least one thermoelectric module or around the plurality of electrical components; and a thermal resistant barrier between the hot side and the cold side.
The moisture resistant barrier around the plurality of electrical components includes a sealant between the power pack heat sink and the mounting frame. A gasket is attached to the cold side of the mounting frame proximate to the power pack cutout. A power pack cover including a base having four sides extending from a peripheral edge of the base to an outer edge, is secured to the gasket. Preferably, a cover seal disposed over the outer edge of the power pack cover.
The moisture resistant barrier around the thermoelectric modules includes a sealant between the hot side heat sink and the mounting frame. A sealing frame having a bottom surface, an outer surface extending from a peripheral edge of the bottom surface, and a free edge formed at a distal end of the outer surface, and a sealing frame opening in the bottom surface, is mounted on the cold side of the mounting frame proximate to the heat sink cutout. A sealant is disposed between the sealing frame and the mounting frame. A sealant is disposed between the free edge of the sealing frame and the cold side heat sink.
The thermal barrier between the hot side and the cold side includes a sealant between the hot side heat sink and the mounting frame. A sealant is disposed between the power pack heat sink and the mounting frame. A gasket is attached to the cold side of the mounting frame proximate to the power pack cutout. A power pack cover including a base having four sides extending from a peripheral edge of the base to an outer edge, is secured to the gasket. Preferably, a cover seal is disposed over the outer edge of the power pack cover. A sealing frame having a bottom surface, an outer surface extending from a peripheral edge of the bottom surface, and a free edge formed at a distal end of the outer surface, and a sealing frame opening in the bottom surface, is mounted on the cold side of the mounting frame proximate to the heat sink cutout. A sealant is disposed between the sealing frame and the mounting frame; and a sealant between the free edge of the sealing frame and the cold side heat sink.
The power pack heat sink has a plurality of blind holes in the base portion of the power pack heat sink, corresponding to a plurality of through holes in the power pack cover. A plurality of fasteners is disposed through the plurality of holes in the power pack cover and secured in the plurality of blind holes in the base portion of the power pack heat sink.
The hot side heat sink has a plurality of blind holes in the base portion of the hot side heat sink, corresponding to a plurality of through holes in the cold side heat sink. A plurality of fasteners is disposed through the plurality of holes in the cold side heat sink and secured in the plurality of blind holes in the base portion of the hot side heat sink. The cold side heat sink may have thermally conductive spacer blocks integrally formed in the cold side heat sink.
The present invention is directed to a thermoelectric cooler including a programmable power control system. The programmable power control system includes a first thermal sensing element for sensing a first temperature on the hot side of the thermoelectric module system. A first input channel is electrically connected to the first thermal sensing element. A second thermal sensing element for sensing a second temperature is located on the hot side of the thermoelectric cooler system. A second input channel is electrically connected to the second thermal sensing element. A processing unit is electrically connected and adapted to read the first temperature from the first input channel and the second temperature from the second input channel. The processing unit calculates a temperature difference, reads from a lookup table to determine an optimum operating voltage based on the temperature difference, and outputs a signal to control power output based on the temperature difference.
The present invention is directed to a method for controlling power to a thermoelectric cooler including the steps of sensing a first temperature on a hot side of a thermoelectric module of the thermoelectric cooler, sensing a second temperature on a hot side of the thermoelectric cooler, inputting the first sensed temperature to a first input channel; inputting the second sensed temperature to a second input channel, reading the first input channel and the second input channel into a processing unit for determining the optimum power output determining a temperature difference between the first temperature and the second temperature, determining an optimum power output for the temperature difference; and outputting a power output control signal.
The present invention is directed to a first DC to DC active power supply in the housing, electrically connected to the least one first thermoelectric module and a second DC to DC active power supply in the housing, electrically connected to the least one second thermoelectric module. Preferably, the first DC to DC active power supply is also electrically connected to the at least on second thermoelectric module; and the second DC to DC active power supply is also electrically connected to the at least on first thermoelectric module.